Rewrite TemplateExpression to enable hierarchical expressions

[MilesCranmer]

• Created ComposableExpression type for building composable mathematical formulas. You can even call these on other expressions to use them like macros.
• Added ValidVector wrapper to handle invalid computation results (e.g., NaN) safely within the TemplateExpression
• Implemented operator overloading for ValidVector to propagate validity flags
• Added vectorized evaluation support for composable expressions
• Added tests for interface, evaluation, and error handling

This enables building complex expressions that can be hierarchical (!).

[github-actions]

Benchmark Results

	master	113f2c6...	master/113f2c6426cdb4...
search/multithreading	23.1 ± 2.8 s	22.6 ± 0.79 s	1.02
search/serial	34.2 ± 0.34 s	34.5 ± 0.2 s	0.992
utils/best_of_sample	2.19 ± 1.7 μs	1.98 ± 1.4 μs	1.11
utils/check_constraints_x10	12.2 ± 3.3 μs	12 ± 3.2 μs	1.02
utils/compute_complexity_x10/Float64	2.09 ± 0.12 μs	2.09 ± 0.11 μs	1
utils/compute_complexity_x10/Int64	2.05 ± 0.11 μs	2.04 ± 0.11 μs	1
utils/compute_complexity_x10/nothing	1.49 ± 0.12 μs	1.48 ± 0.1 μs	1.01
utils/insert_random_op_x10	5.98 ± 1.8 μs	5.71 ± 1.8 μs	1.05
utils/next_generation_x100	0.44 ± 0.068 ms	0.348 ± 0.12 ms	1.26
utils/optimize_constants_x10	0.0398 ± 0.0076 s	0.0401 ± 0.0097 s	0.992
utils/randomly_rotate_tree_x10	5.33 ± 0.6 μs	5.26 ± 0.58 μs	1.01
time_to_load	1.81 ± 0.037 s	1.84 ± 0.044 s	0.981

Benchmark Plots

A plot of the benchmark results have been uploaded as an artifact to the workflow run for this PR.
Go to "Actions"->"Benchmark a pull request"->[the most recent run]->"Artifacts" (at the bottom).

[coveralls]

Pull Request Test Coverage Report for Build 11717823949

Details

• 241 of 248 (97.18%) changed or added relevant lines in 15 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.02%) to 95.422%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/LossFunctions.jl	12	13	92.31%
src/Operators.jl	2	8	25.0%

Files with Coverage Reduction	New Missed Lines	%
src/SearchUtils.jl	1	90.4%
src/Dataset.jl	1	92.86%

Totals
Change from base Build 11599648113:	-0.02%
Covered Lines:	3022
Relevant Lines:	3167

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💛 - Coveralls